The invention relates to a high critical temperature (HTc) superconductive multifilament strand and to a method of making such a strand. More particularly, the invention relates to an HTc superconductive multifilament strand clad in silver and used with AC, and to a method of making such a strand.
The use of HTc superconductive multifilament strands with AC causes power losses due to induced currents. It is known to reduce such losses by twisting the conductor at a very small pitch with a very small filament diameter. However, that is effective only if the filaments are electrically decoupled from one another by means of a resistive barrier.
It is known to make HTc multifilament strands by the xe2x80x9cpowder in tubexe2x80x9d technique. That consists in filling a billet with powder reagents that are suitable, after heat treatment, for transforming into a superconductive material, and in particular into a material of the HTc ceramic type.
The billet is then closed under a vacuum and drawn down, after which it is put into a bundle in a new billet, itself in turn closed under a vacuum and then drawn down. The resulting multifilament strand may be subjected to the same steps, and so on until a desired number of filaments per unit area has been obtained.
The strand made in this way is then put into its final form, e.g. by rolling and/or twisting, and is then subjected to heat treatment to transform its powder reagents.
The material constituting the billets must be sufficiently ductile to be capable of withstanding the various drawing-down and rolling stages, and its composition must be inert or at least without consequence for the heat treatment that transforms the powder reagents into a superconductive phase. It is known that silver can be used as the material constituting the billets.
However, silver is a material that is very highly conductive at the operating temperatures of HTc super-conductors. As a result there is practically no electrical decoupling between the filaments.
It is known that Ag can be doped with impurities of the Pd or Au type to 1% or 2%. That technique makes it possible to gain two decades in resistivity at 20 K.
However, the Ag/Pd alloy is expensive which makes it economically inconceivable in mass production applications.
It is also known to make at least one multilayer composite billet comprising at least one outer thickness of metal alloy, at least one of whose components is oxidizable, and an inner thickness of silver.
Heat treatment is then applied so that the oxidizable components of the metal alloy diffuse at the interface between the metal alloy and the silver and, in the presence of oxygen or of oxygen-containing compounds, oxidize to form an insulating oxide at said interface.
Filament decoupling is thus significantly improved. However, making such an oxide barrier consumes oxygen, thereby preventing the precursors from being properly synthesized into the superconductive phase.
To mitigate that problem, it has been proposed (Y.B. HUANG, R. FLUKIGER, in an article presented to SPA""97, Mar. 6-8, 1997, XU""AN, China) to use as an insulating barrier a compound that behaves in the same way as the precursors, i.e. that is permeable to oxygen while being a poor superconductor at the operating temperatures of HTc multifilament strands. Such a compound is known, e.g. Bi2202 which is not a good super-conductor, or Bi2212 degenerated by Al, Mg, or Ti pollution. With Bi2212, SrAl2O4 or CaAl2O4 is created, for example, thereby departing from the stoichiometry necessary to have a Bi2212 superconductive phase.
One of the objects of the present invention is to propose a method that uses the latter approach and that enables a multifilament strand to be made that is very long and in which filament decoupling is significantly improved, while retaining good permeability to oxygen in the non-conductive layer for the purposes of synthesizing precursors into the superconductive phase.
To this end, the invention provides a powder in tube type method of making an HTc superconductive multifilament strand having a silver-based matrix, in which method:
in a monofilament step, a first silver-based envelope is filled with powder reagents suitable, after heat treatment, for transforming into an HTc super-conductive material;
the resulting billet is drawn down into a monofilament strand;
in a first multifilament step, said monofilament strand is cut into lengths and a secondary silver-based envelope is filled with the resulting lengths, thereby making a multifilament billet, the multifilament billet being drawn down in turn to form a multifilament strand;
in a secondary multifilament step performed at least once, said multifilament strand is cut into lengths and a new silver-based envelope is filled with the resulting lengths, thereby making a new multifilament billet, the new multifilament billet being drawn down in turn to form a new multifilament strand;
the new multi-filament strand is formed; and
heat treatment is applied to the formed strand;
according to the invention, in a step performed prior to the monofilament step:
a composite multilayer material is prepared using a method derived from the method described in European Patent Document EP 0 531 188 dated Dec. 11, 1996, the composite multilayer material comprising at least one silver-based sheet, and at least one layer of non-superconductive ceramic material that is permeable to oxygen; and
during the mono-filament step, a thickness of composite multilayer material is interposed between first and second thicknesses of silver-based material, thereby forming said first silver-based envelope.
In order to improve the performance of the multifilament strand further, the cross-sections of the first silver-based envelope, of the monofilament strand, of the secondary silver-based envelope, of the multifilament strand, of the new silver-based envelope, and of the new multifilament strand are preferably square or rectangular in overall shape, although any other shape, e.g. round or hexagonal, is also acceptable.
In one implementation, to prepare the composite multilayer material:
a polymer is mixed with a powder that is suitable, after heat treatment, for transforming into a non-superconductive ceramic material that is permeable to oxygen, mixing taking place at a temperature. approximately equal to the melting point of the polymer in question so to obtain a homogeneous polymer-and-powder mixture;
at least one sheet of silver is hot compressed with the polymer-and-powder mixture at a temperature approximately equal to the melting point of the polymer in question so as to obtain an intermediate composite multilayer material comprising silver alloy and homogeneous polymer-and-powder mixture; and
the composite multilayer material comprising silver alloy and homogeneous polymer-and-powder mixture is baked at approximately the melting point of the powder to remove the polymer, to make the powder synthesize into non-superconductive ceramic that is permeable to oxygen, and to make the non-superconductive ceramic that is permeable to oxygen adhere to the sheet of silver, thereby making the composite multi-layer material.
Provision may be made to hot compress the homogeneous polymer-and-powder mixture between two silver-based sheets.
The thickness of the layer of-homogeneous polymer-and-powder mixture is chosen as a function of the thickness desired for the oxygen-permeable non-superconductive ceramic around the monofilaments of the formed new multifilament strand.
In one implementation, the powder is based on Bi2212 polluted with Al, Mg, or Ti, for example.
In another implementation, the powder is based on Bi2201.
The invention further provides an HTc superconductive multifilament strand comprising a plurality of superconductive filaments, each superconductive filament comprising an HTc superconductive ceramic core surrounded by first cladding of an Ag-based alloy, itself surrounded by a layer of non-superconductive ceramic that is permeable to oxygen, itself surrounded by second cladding of an Ag-based alloy.
A first advantage of the present invention results from the fact that the compositions of the precursors and of the non-superconductive ceramic are similar. This mitigates any problem of chemical incompatibility.
Another advantage of the present invention results from the fact that the mechanical and/or heat treatment steps of the method of the invention are unchanged compared with conventional steps. This means that existing installations can be used without it being necessary to adapt them.
Compared with silver, the ceramic barrier offers resistivity gains of about 103 at 300 K and about 105 at 20 K.